Directional drilling has become commonplace in the exploration and development of oil and gas fields throughout the world. Yet, directional drilling remains a complex science focused upon the control of a rotary drill's angular and rotational characteristics in such a manner as to allow the borehole to be drilled in conformity with a pre-established well plan. The ability to predictably reach a specific subsurface target enhances the likelihood of successful well completion and greatly reduces the expense of oil and gas exploration.
Numerous forces work upon the drill string and drill bit to induce deviation from the intended drilling plan. These forces vary with the weight applied to the drill bit, the type of formation being penetrated, the speed of revolution of the drill bit, and the angle of the borehole. Drill string stabilizers are utilized to limit the effect of forces on the direction of the bore hole by increasing the rigidity of the lower portion of the drill string. In directional drilling, drill string stabilizers serve to aid in attaining the desired deviation in the borehole angle and direction or in maintaining the borehole angle and direction once attained.
The advent and improvement of the down-hole motor has fostered the use of down-hole equipment containing an angular deviation which facilitates the "kick-off" of the bore-hole in a new direction. The down-hole motor provides for rotation of the drill bit while the drill string is held in a constant configuration, resulting in a "controlled tool-face." In this manner the borehole follows the angular deviation placed in the down hole equipment since the angle is maintained when only the drill bit rotates. Once the desired amount of deviation of the borehole is acheived, the entire drill string may be rotated which negates the angular deviation found in the down-hole equipment. This may result in excessive vibration and borehole "washout," since rotation of the angled portion of the down hole equipment can create a borehole diameter larger than the drill bit itself. Washout is generally minimized by the small angular deviations usually employed, often less than one degree.
If desired, washout can be avoided by tripping the drill string and removing the angled down-hole equipment (the "bent-sub" or "bent-housing mud motor") prior to rotation of the entire drill string. However, tripping the the drill string is a time consuming and expensive process. The recent development of down-hole equipment which can be adjusted from a bent or angled mode to a straight mode while still in the borehole (such as "hydraulic bent-subs") avoids washout and facilitates changing bore-hole direction or angular deviation by allowing the adjustment of the bend angle of the down-hole equipment without tripping the drill string. After use of the down-hole motor, the down-hole equipment is adjusted from a bent mode to a straight mode while still in the borehole, and drilling is continued by rotating the entire drill string.
Drill string stabilizers can play a vital role in drilling procedures using bent-subs or bent-housing mud motors. As shown in U.S. Pat. No. 4,667,751, (Geczy) the placement of concentric stabilizers along the drill string has an effect upon the deviation of the borehole. The variation of the placement of stabilizers along the drill string will result in differing borehole deviations during directional drilling. As shown in U.S. Pat. No. 4,384,626 (Derouin), the use of clamp-on stabilizers enables the placement of stabilizers at any position along the drill string, avoiding the limitations of placement inherent in stabilizers which form a part of a threaded pipe section that is made up in the drill string itself.
Yet, the advent of bent-subs and bent-housing mud motors, and the ability to adjust the angular deviation of of this equipment without tripping the drill string has created a need for new types of stabilizers which has not been addressed by the prior art. During the use of the down-hole motor the drill string does not rotate, resulting in a "controlled tool face." The down-hole equipment tends to rest upon the low side of the borehole, which is the side of the borehole closest to a true vertical bore. The drill bit can be supported and directed by a stabilizer placed near the drill bit. However, the stabilizers addressed by the prior art cannot run "full-gage," (with the same diameter as the borehole created by the drill bit), without limiting the angular and directional changes desired by the use of a bent-sub or bent-housing mud motor. Accordingly, stabilizers that run less than "full-gage" are recommended by the prior art. (See U.S. Pat. No. 4,384,626 (Derouin) at FIG. 6; and U.S. Pat. No. 4,667,751 (Geczy).
After the desired angular and directional deviation of the borehole is achieved through the use of the down-hole motor, the entire drill string is rotated. Stabilizers which are attached to, or form a part of the drill string also rotate. It can readily be seen that any stabilizer which has bore-wall bearing surfaces that are not equidistant from the axis of rotation of the drill string, or which does not run "full-gage" will not act to maintain the drill string in the center of the bore-hole. This is true of the stabilizers described in U.S. Pat. No. 4,384,626 (Derouin FIG. 6) and in U.S. Pat. No. 4,667,751 (Geczy).
Several inventions akin to stabilizers have a supporting side which seeks the low side of the borehole due to the weight of the support section. These devices purport to maintain their orientation with the low side of the bore-hole even when the entire drill string is rotated, through the use of a mandrel and sleeve arrangement. Examples of these inventions are found in U.S. Pat. No. 4,638,873 (Welborn) and U.S. Pat. No. 4,220,213 (Hamilton). These devices suffer from problems associated with the complexity of the mandrel and sleeve arrangement.